1. Field of the Invention
This invention relates to a novel gate structure for an integrated circuit comprising elements of the gate-insulator-semiconductor types and also relates to a method of fabrication of an integrated circuit which makes use of said structure.
2. Description of the Prior Art
Known circuits which employ elements of the type just mentioned include in particular the charge-transfer devices as well as circuits using elements of the MOS type (metal-oxide-semiconductor elements). Circuits in this class are now well-known to those versed in the art and are described in particular in the book by C. H. Sequin and M. F. Tompsett entitled "Charge Transfer Devices" or in the article by J. L. Berger and al. Which was published in the March 1980 edition of "Revue Technique THOMSON-CSF" under the heading "Les dispositifs a transfert de charge". In the circuits mentioned in the foregoing, the gates are made of conductive material which can be either a metal such as aluminum or a metalloid such as strongly doped polycrystalline silicon. However, the techniques of fabrication of integrated circuits comprising elements of the gate-insulator-semiconductor type in which the gate is formed in particular by a layer of polycrystalline silicon do not permit the fabrication of a gate of small thickness.
This problem is particularly objectionable in the case of photosensitive semiconductor devices in which the photosensitive elements or photodetectors are of the gate-insulator-semiconductor type such as, in particular, the charge-transfer devices in which light radiation is directed to charge-transfer registers, and the photosensitive devices of the type designated as CID (Charge Injection Devices) constituted by two MOS capacitors coupled by a gate in which one of the capacitors is subjected to the light radiation to be detected. In fact, in devices of this type, when the front face of the photosensitive device is illuminated, the photons have to pass through the gate and the insulators of the capacitor which forms the photodetector before arriving in the semiconductor material in order to form electron-hole pairs in this latter. The layers referred-to above must therefore be fabricated from materials which are transparent to the radiations to be integrated. Thus in the case of integrated circuits in which the substrate is of silicon, the gate is usually formed of semi-transparent polycrystalline silicon. As mentioned earlier, however, it is difficult to form a gate of small thickness with a material of this type. This result in high attenuation of the light rays which pass through the gate, particularly in the case of blue wavelengths.